1. Field of the Invention
The present invention generally relates to a radiation detection circuit and an apparatus for radiographic examination, and more particularly relates to a radiation detection circuit and an apparatus for radiographic examination that detect two gamma rays emitted from a radioactive isotope in the object at the same time.
2. Description of the Related Art
Positron emission tomography apparatuses are being used to obtain detailed information on the object. Before a diagnosis using a PET apparatus, diagnostic agents labeled by positron nuclides are introduced into the object by injection or inhalation. The diagnostic agents introduced into the object accumulate in a body part having a function corresponding to the diagnostic agents. For example, diagnostic agents made of saccharide accumulate preferentially in a part of the object where metabolism is high, for example, cancer cells. The positron nuclide of the diagnostic agent emits a positron. When the emitted positron collides with one of surrounding electrons, both are annihilated and two gamma rays are emitted at approximately 180 degrees to each other. The two gamma rays are detected at the same time by gamma ray detectors surrounding the object and recorded as signals. A computer processes recorded signals and generates image data showing the distribution of radioactive isotopes in the object. While a computer tomography (CT) scanner used for detailed diagnosis provides structural information on a lesion in the object, a PET apparatus provides functional information on the inside of the object and therefore makes it possible to clarify the pathologies of various intractable diseases.
A PET apparatus determines that the signals are valid only when two gamma rays emitted from a positron nuclide at approximately 180 degrees to each other are detected at the same time by a pair of gamma detectors facing to each other across the object. For example, when only one gamma ray is detected at a time, the signal is discarded as invalid. Even when two gamma rays are detected at two close time points, the signals are discarded as invalid if the time difference between the two time points is greater than a specified value. For the above reasons, it is necessary to precisely determine the time at which a gamma ray enters a gamma ray detector.
A gamma ray detection circuit obtains the time (detection point) of a detected signal the pulse height of which detected signal increases in the time axis direction, and uses the detection point as the incidence time of the gamma ray. Various types of detection circuits for determining the detection point of a detected signal have been proposed. For example, a detection circuit uses the time at which a detected signal reaches a specified pulse height as the detection point. An advantage of such a detection circuit is that the circuit configuration is simple. However, obtained detection points may fluctuate depending on the maximum pulse heights of detected signals or depending on the waveforms of detected signals.
In a detection circuit 100 as shown in FIG. 1, a zero-cross comparator 102 compares a signal Yz obtained by dividing the voltage of a detected signal by voltage dividing resistors R1 and R2 with a signal Xz obtained by delaying the detected signal for a specified period of time by a delay circuit 101. The detection circuit 100 then generates an output signal using the zero-crossing time as a detection point. Such a circuit is called a constant fraction discriminator (CFD) (see, for example, non-patent document 1).
A CFD outputs a pulse when a certain period of time passes after a detected signal reaches a certain pulse height, determines the time at which the pulse has been output, and uses the time as a detection point. A CFD can determine a detection point independently of the pulse height itself of a detected signal, thereby reducing the fluctuation of detection points.
[Non-patent document 1] 2003 IEEE-Nuclear Science Symposium, Integrated Circuit Front-Ends for Nuclear Pulse Processing: Short Course “Front-end Circuits for Timing Applications” by Alan Wintenberg
However, since the delay circuit 101 of the detection circuit 100 shown in FIG. 1 is normally formed by connecting many operational amplifiers, the configuration of the detection circuit 100 is complicated.
To increase the positional accuracy and efficiency of gamma ray detection in a PET apparatus, it is necessary to miniaturize a detector and thereby to arrange a large number of detectors in the PET apparatus. Increasing the number of detectors makes it necessary to increase the number of detection circuits. Therefore, in practice, it is necessary to form detection circuits on a semiconductor chip. Also, depending on the characteristics of a detector, it may be necessary to adjust the delay time of the delay circuit 101 shown in FIG. 1. However, adjusting the delay time requires changing the number of operational amplifiers and the design of a semiconductor chip, and therefore requires rebuilding the semiconductor chip. This results in increased production costs and increased production time of a PET apparatus. Further, similar problems occur when changing the design of a detector or when using detectors with different characteristics.